Magnetic screening system

ABSTRACT

A magnetic screening system uses directional gradiometers with high resolution and accuracy to measure magnetic field signatures of target objects (e.g., gun, knife, cell phone, keys) in a volume of interest. The measured signatures can be compared to signatures of known objects stored in a local database. Various mathematical processes may be used to identify or classify target object signatures. In a network of magnetic screening systems, the magnetic screening systems can transmit signatures to a central signature database, and a management computer can share the central signature database with all of the magnetic screening systems on the network. The magnetic screening system can operate in multiple modes, such as a tracking mode, measurement mode, and self-test mode. Through use of unique processes and designs, the magnetic screening system can achieve a high rate of processing persons for target objects.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/073,424, filed Mar. 4, 2005. The entire teachings of the aboveapplication are incorporated herein by reference.

BACKGROUND OF THE INVENTION

In recent years, screening for weapons at entrances of public places,such as airports, government buildings, public schools, and amusementparks, has increased to ensure safety for the public at those places.Screening for weapons can include requiring people entering such publicplaces to pass through a magnetic screening system such as a portalmetal detector. Although people have become accustomed to passingthrough portal metal detectors, the process remains relatively slow fora number of reasons.

One reason for the slow process is that people must empty their pocketsof all metallic objects, remove their coats, and sometimes remove theirshoes. The objects and clothing are either physically inspected by handor passed through an x-ray machine for visual screening. Another reasonfor the slow process is due to false detection or detection ofnon-weapon metals, such as wrist watches, belt buckles, metallic wiresfound in ordinary garments, personal adornments such as broaches or hairclips, and loose coins in pockets.

Yet another reason for the slow processing is due to machine settlingtimes, which refers to the amount of time that must be allowed for thesensors in metal detectors to resettle after a person passes through it.Allowing a portal metal detector sufficient time to settle ensuresaccurate readings of the next person. A person who passes through theportal metal detector then stops on the other side very close to theportal metal detector (i.e., within an ‘influence’ zone) can alsoinfluence the metal detector to such an extent that the metal detectormakes a false detection or misses detecting an object as the next personpasses through it. Therefore, portal metal detector operators must stopthe next person from entering the portal metal detector until theprevious person has passed beyond the influence zone.

Typically, all of the delays result in a passthrough rate of between oneand two hundred persons per hour. To accommodate large crowds, manyportal metal detectors are operated in parallel, which leads tostaffing, training, and machine calibration issues. If the passthroughrate were higher, many venues that are currently equipped with largenumbers of portal metal detectors could reduce the number in use, andvenues such as sports stadiums not currently equipped with portal metaldetectors would be so equipped.

Moreover, today's portal metal detectors are sensitive to largeferromagnetic objects, such as wheelchairs. When a person in awheelchair passes through the portal metal detector, the portal metaldetector is overwhelmed by the metal content of the wheelchair andunable to detect relatively small metal objects on the person. Inaddition, even if the wheelchair is not passing through the portal metaldetector, it can influence the detector to such an extent that thedetector makes erroneous readings.

In addition to the slow process associated with today's portal metaldetectors and their sensitivity to large ferromagnetic objects, manyportal metal detectors are ‘active,’ meaning they emit anelectro-magnetic field in a volume of interest (i.e., the area in theportal metal detector). Active detectors can be dangerous for peopleusing medical devices, such as pacemakers, that are sensitive toelectro-magnetic fields. Passive metal detectors, which sense a localdisturbance in the earth's magnetic field, do not affect medicaldevices, but they are sensitive to local magnetic fields, largeferromagnetic devices, calibration errors, background offsets, and othermeasurement disturbances known in the art.

SUMMARY OF THE INVENTION

The principles of the present invention apply to multiple levels of amagnetic screening system and a network managing multiple magneticscreening systems. In a network embodiment, signature data (“signatures”or data) of target objects (e.g., guns, knives, cell phones, PersonalDigital Assistants (PDA's), and other ferromagnetic materials) andinformation associated therewith can be added to and maintained in acentral signature database. From this central signature database, amanagement computer can distribute the information and data in thecentral signature database to magnetic screening systems on the networkfor updating their local databases. As a result, the local databases canbe continually updated, thereby allowing the magnetic screening systemsto have knowledge of more target objects than if operating independentlyoff the network for increased automation of detection, identification,or classification processes, among others, of target objects. Increasedautomation increases a rate of processing of people through the magneticscreening systems because operators of the magnetic screening systemshave fewer incidents of having to manually inspect persons carryingtarget objects.

The magnetic screening systems may include arrangements of gradiometers,each including at least three magnetometers and, in some embodiments, agradiometer processor in communication with the magnetometers. Thegradiometer processor scales outputs from the magnetometers with unequalweights and combines the scaled outputs to orient a direction ofsensitivity of the respective gradiometer toward a volume of interest(e.g., a pathway through a portal metal detector). The magneticscreening systems may also include an arrangement in communication withthe gradiometer that uses the gradiometers in a collective manner todetect a target object. In addition to detecting a target object, thearrangement processor can localize the position of the target object,identify the target object, and optionally classify the target object.The gradiometers may use passive magnetometers, which do not themselvesgenerate a magnetic field, thereby allowing people with medical devices,such as heart pacemakers, to pass through the magnetic screening system.The arrangement processor may also be used to compare a signature of thetarget object measured by the gradiometers against known signaturesstored in a local or central database.

The gradiometers may be operated in multiple modes. Examples of modes inwhich the gradiometers may be caused to operate include measurementmode, background offset reduction mode, calibration mode, self-testmode, automatic alignment mode, and diagnostic mode. Self-test mode canbe used to determine operational readiness. Automatic alignment mode canbe used to calculate the alignment of the gradiometers relative to theearth's magnetic field, which, in turn, can be used to determineorientations of each gradiometer to at least one other gradiometer inthe magnetic screening system. Knowing alignment of gradiometers allowsfor system operation in a tracking mode, in which multiple gradiometerscan be configured to generate real-time tracks of target objects inthree dimensions. In diagnostic mode, the gradiometers can outputmeasured field strengths in an unaltered state from the componentmagnetometers. During measurement mode, the gradiometers can be switchedfrom measurement mode to calibration mode or background offset reductionmode in various sequences and at selectable rates. Background offset,caused by disturbances within or outside a volume of interest thataffect measurements by the gradiometers, can be reduced in a real-timemanner or in a post-processing manner, and the magnetometers can becalibrated before every measurement sample. Use of the above-describedmodes can yield accuracies that result in minimized rates of falsedetection of known or unknown target objects, including high accuracy inautomatically determining whether a target object is a dangerous objector a non-dangerous object. Thus, processing rates of persons passingthrough the magnetic screening system(s) is increased.

To increase calibration accuracy, the gradiometers may have individualcalibration circuits available for applying localized magnetic fields totransducers in the magnetometers that cause a measurable response by themagnetometer. During a calibration cycle, the calibration circuit may beused to generate magnetic fields at least two different levels. Thecalibration circuit may be specially designed to limitexternally-induced offsets for improved calibration accuracy. Using thecalibration circuit, the gradiometer processor or other processor cancalculate a calibration curve using various techniques or metrics. Thecalibration curve can be used to calibrate every magnetic field vectorsample measured by the respective magnetometer. This calibrationcircuit, thus, improves measurement accuracy, which, in turn, reducesrates of false alarm for increased rate of processing persons throughthe magnetic screening system(s).

As described above, the gradiometers may each include at least threemagnetometers whose outputs are scaled with unequal weights. The unequalweights may be combined to orient the direction of sensitivity of thegradiometer toward a volume of interest. In one embodiment, the weightsare non-integer weights that may be calculated using a deterministicmathematical technique. In some embodiments, a processor may adjust theoutput weights digitally and optionally in real-time. Spacing of themagnetometers can improve accuracy of the gradiometers. For example, inthe case where the magnetometers are aligned along a single axis, theouter two magnetometers are preferably spaced apart as far as possiblefor enhanced sensitivity for measuring a magnetic gradient. Themagnetometer(s) between the outer two magnetometers may be arbitrarilypositioned relative to the outer two magnetometers to orient thedirection of sensitivity toward the volume of interest. Optionally,positioning of the magnetometers is determined before the output weightsare determined. In some embodiments, the direction of sensitivity isentirely toward the volume of interest, and the sensitivity away fromthe volume on interest is substantially zero. In such an embodiment,gradiometers positioned on a first boundary of the volume of interest donot detect disturbances behind themselves outside the volume of interestbelow a selectable threshold, such as a wheelchair or other magneticscreening system. The disturbance outside the volume of interest can bedetected by gradiometers on the other side of the volume of interestand, thus, be treated as a background disturbance and eliminated frommeasurements. Again, such processing techniques and designimplementations are used to reduce rates of false detection, improveaccuracy, and ultimately lead to increased rate of processing of personspassing through the magnetic screening system(s).

As a result of the multiple aspects of the present invention asdescribed above, a magnetic screening system can process 600-700 personsper hour or more compared to previous systems capable of processing100-200 persons per hour. Moreover, the magnetic screening systems canmake the screening process much more acceptable for people since (i)jackets and other typical outerwear can be worn while passing throughthe magnetic screening system and (ii) ferromagnetic objects, such askeys, cell phones, personal digital assistants, and other ferromagneticobjects do not have to be removed from pockets as a result of thetechniques described herein.

Moreover, in some embodiments, arrangements of gradiometers can bedeployed in fixtures other than portals, such as wastebaskets or othercommon fixtures, so as to be imperceptible to persons passing through avolume of interest defined by placement of the arrangement(s) ofgradiometers. Thus, passive metal detection can be done by the magneticscreening systems that do not disrupt traffic flow while yielding highrates of detection of dangerous target objects (e.g., guns, knives, andso forth) and discriminating non-dangerous target objects (e.g., cellphones and the like).

As the magnetic screening system(s) are used, the database of signaturescorresponding to known ferromagnetic objects increases and, therefore,improves the overall operation of the system even more over time.

As a result of the improvements described herein, the magnetic screeningsystems may be employed at venues such as sports stadiums, amusementparks, and other public places in which such systems were previouslythought to be too restrictive on a flow-through basis or publicrelations basis. At venues where magnetic screening systems arecurrently used in high numbers, such as airports, the number of magneticscreening systems can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

FIG. 1 is a pictorial diagram of an entrance to a public place havingmagnetic screening systems (e.g., portal metal detectors) employing theprinciples of multiple aspects of the present invention;

FIG. 2 is a network diagram of a magnetic screening system networkincluding the magnetic screening systems of FIG. 1;

FIG. 3 is a network block diagram corresponding to a subset of thenetwork of FIG. 2;

FIG. 4A is a block diagram of a gradiometer used in the magneticscreening systems of FIG. 1.

FIG. 4B is an electrical schematic diagram of the gradiometer of FIG.4A;

FIG. 5 is an electrical schematic diagram of a magnetometer withtransducer used in the gradiometer of FIG. 4B;

FIG. 6A is an electrical schematic diagram of the transducer of FIG. 5and an associated calibration circuit;

FIG. 6B is a plot of data points captured by a calibration process usingthe calibration circuit of FIG. 6A;

FIG. 7A is a magnetic pattern diagram for the gradiometer of FIG. 4A.

FIG. 7B is a detailed lobe diagram for the gradiometer of FIG. 7A;

FIG. 7C is a lobe diagram for a prior art three-magnetometergradiometer;

FIG. 7D is a detailed lobe diagram for the prior art, three-magnetometergradiometer of FIG. 7C;

FIG. 8 is a vector diagram for the gradiometer of FIG. 7A measuring atarget object;

FIG. 9 is a timing diagram for multiple modes of operation for thegradiometer of FIG. 8;

FIG. 10 is timing diagram for the system operation diagram of FIG. 9.

FIG. 11A is a graphical diagram illustrating a real-world example forthe magnetic screening system of FIG. 9;

FIG. 11B is a detailed timing diagram corresponding to the real-worldillustration of FIG. 11A;

FIG. 12A is graphical illustration of target object having magneticfields detectable by the magnetic screening system of FIG. 11A;

FIG. 12B is an alternative embodiment of the magnetic screening systemof FIG. 12A using gradiometers in a tracking mode to track targetobjects in three dimensions;

FIG. 12C is an alternative embodiment of the magnetic screening systemof FIG. 12A in which an arrangement of gradiometers is deployed innon-portal fixtures and can operate in a tracking mode to track targetobjects in three dimensions;

FIG. 13 is an example signature for a target object (e.g., a gun)captured by the gradiometer of FIG. 4A;

FIG. 14A is a signal diagram illustrating the signature of FIG. 13 in ameasurement signal affected by a background disturbance and captured bythe gradiometer of FIG. 4A;

FIG. 14B is the signal diagram of FIG. 14A with a background offsetremoved either in real-time or during post processing;

FIG. 15 is a block diagram of identification and classificationprocessing used to identify or classify the target object signature ofFIG. 13; and

FIG. 16 is a graphical diagram illustrating the magnetic screeningsystem of FIG. 1 that is presenting an indicator produced by theprocessing of FIG. 15 to a portal metal detector operator.

DETAILED DESCRIPTION OF THE INVENTION

A description of preferred embodiments of the invention follows.

The principles of the present invention may be employed at multiplelevels in the magnetic screening system 100 and a network of magneticscreening systems 100 each including multiple gradiometers. The multiplelevels include at least a network level, system level, mode of operationlevel, and gradiometer design level. A brief overview of each ispresented, and details of each, in turn, follows the brief overview.

FIG. 1 is a pictorial diagram of a cluster 50 of magnetic screeningsystems 100, each using gradiometers configured in a portal 105.Magnetic screening systems 100 are typically used at an entrance to apublic place, such as an airport, government building, public school, oramusement park. The magnetic screening systems 100 are used to ensuresafety for the public. The magnetic screening systems can identifymagnetic objects, referred to herein as “target objects” on a person, asunderstood in the art. In this example embodiment, each magneticscreening system 100 includes the portal 105 (more generally referred toherein as an arrangement of gradiometers), an operator station 110, anda screening computer 115.

A traditional magnetic screening system configured with a portal iscapable of processing approximately 100-200 persons per hour. Throughuse of the principles of one or more aspects of the present invention,the number of persons the portal metal detectors can process increasesto between 600 and 700 persons per hour or more. There are many reasonsfor the increase in processing rate. One reason is that the peoplepassing through the portals 105 do not need to empty their pockets ofmetal objects. In some embodiments, the portals 105, screening computers115, operator stations 110 can detect, identify, and classify metalobjects being carried by people. For example, processing in the magneticscreening systems 100 can determine that target objects are a cellphone, loose change, brassier underwire, shoe support, wristwatch, hairclip, or other non-dangerous weapon in an automated manner and excludethese objects from causing an alert signal to an operator. Additionally,dangerous weapons, such as knives or guns, can be detected, identified,or classified through use of the processing according to the principlesof the present invention.

Another reason for the higher processing rate of people passing throughthe magnetic screening systems 100 is due to a low rate of falsedetection. The reason for a low rate of false detection is because themagnetic screening systems 100 can remove background offset before,during, or post measurement acquisition. The magnetic screening systems100 can also perform calibration during measurement, optionally at ahigh rate, which improves accuracy of the measurements. Further, becauseof the removal of background offset from the measurements, persons whodwell in an “influence zone” after passing through the portal metaldetector 105 does not adversely affect measurements of the next personto pass through the portal 105.

Another reason for the portal metal detector system 100 is able toperform processing at a higher rate over traditional processing isbecause it can force resettling of gradiometers in an arrangement ofgradiometers that are inside the portal 105. Typical portal metaldetectors rely on “drift to zero” for resettling before taking the nextmeasurement without an offset. In the magnetic screening system 100according to the principles of the present invention, the gradiometerscan be driven to their “resettling” state in a forced manner.

Because of the technical advantages provided by the principles of thepresent invention, the magnetic screening systems 100 can process asingle person or multiple persons passing through at the same time,which can save a tremendous amount of time. It should be noted that the600-700 person per hour processing rate does not include the additionalgain of having multiple persons pass through the portal 105, forexample, at the same time.

Also, because of the ability to reduce offset, large ferromagneticsources of disturbances are allowed to pass through the portal 105 thatare typically unable to pass through portal metal detectors. Forexample, wheelchairs can pass through without deleterious effect on theperformance of the magnetic screening system 100. This ability toprocess wheelchairs includes an ability to allow the wheelchair to passthrough the portal 105 or be nearby or within a given area inside avolume of interest 75 defined by the portal 105.

In addition, the gradiometers used by the magnetic screening system 100inside the portal 105 are electromagnetically passive. This means thatthey do not project magnetic fields which are known to cause problemsfor persons using medical equipment that is sensitive to magneticfields, such as a cardiac pacemaker. Other sensitive electronicequipment is also be able to pass through without any adverse effectsince the gradiometers can be passive gradiometers.

At the network level, a management computer stores information and datacommon to all magnetic screening systems. The management computer mayhave or receive common information and data from clusters or individualmagnetic screening systems and share the stored information or data withother clusters or individual magnetic screening systems via acommunications network, such as the Internet. An example of data storedin the common database is signature data for target objects, which canbe any ferromagnetic objects (e.g., gun, knife, cell phone, keys). Anexample of information stored in the database includes an indicator(e.g., graphic or photograph of a gun) or description associated withthe signature.

At the system level, the principles of the present invention may be usedto detect, identify, or classify ferromagnetic objects through use of anarrangement of gradiometers (i.e., magnetic sensors). The architectureof the magnetic screening systems allows their response to a particularset of target objects to be tailored in accordance with sets of targetobjects that operators choose to include or exclude. These targetobjects may be objects that are threats to safety, such as guns andknives, or any other ferromagnetic object that may pose a risk ofinjury. The magnetic screening system may use a variety of signalprocessing techniques in combination to detect, localize, identify, orclassify the ferromagnetic objects. The technique for detecting theferromagnetic objects may be completely electromagnetically passive, incontrast to similar devices that generate an active magnetic field.

At the mode of operation level, each gradiometer utilizes at least threevector magnetometers of any underlying technology to measure a magneticfield gradient. Typically, the magnetic field being measured is theearth's magnetic field, but may be a different, man-made or biologicmagnetic field, such as found in medical applications. The multi-modalgradiometer includes signals and processing methods to provide highrate, high dynamic range signals. The multi-modal gradiometers includemodes, such as measurement mode, automatic offset reduction mode,calibration mode, tracking mode, self-test mode, automatic alignmentmode, and diagnostic mode, and may also include a method to eliminatehysteresis in the measurements.

At the gradiometer design level, the principles of the present inventionoptimize a response of the gradiometers. In particular, the principlesof the present invention relate to a method and corresponding apparatusfor optimizing near and far field responses of a plurality of vectormagnetometers in the gradiometers so as create a field response patternoptimized for a particular sensing application. The method utilizes theindividual magnetometers applied in a certain arrangement to shape thefield response. Additionally, data from magnetometers in the preferredarrangement is processed so as to enhance shaping of the field responsepattern. In particular, the field response pattern can be used to reduceeffects of extraneous and background signals that may be undesirable oroverwhelming to the signal of interest.

The description that follows provides details of the multiple aspects ofthe principles of the present invention.

The magnetic screening system 100 detects, selectively identifies,localizes, or excludes an item from a volume of interest. This isaccomplished by utilizing an arrangement of high-order magneticgradiometers (i.e., at least three magnetometers 400) used in acollective manner combined with global processing of all the arrangementelements to detect, localize, or identify the target objects. In otherembodiments, a gradiometer may include one or two magnetometers, andprocessing may allow an arrangement of gradiometers to be used asdescribed in reference to gradiometers with three magnetometers or areduced set of processing may be employed for more limited detection oftarget objects. The detection method may be entirely electromagneticallypassive.

One advantage resulting from the use of high order gradiometers is thatthe effect of nearby ‘hard iron’ and moving ferromagnetic objects can besuppressed. The use of a totally passive sensor system allows fordiscreet screening of a volume of interest without the knowledge of theperson or persons passing through the volume of interest. The passiveembodiment of the magnetic screening system also eliminates apossibility of interference with life sustaining devices, such ascardiac pacemakers, and does not interfere with the operation ofpersonal electronic devices, such as cell phones. Another advantage ofthis aspect of the present invention is that signal processing elementstasked with processing measurements by the gradiometers can be remotelylocated anywhere there is a suitable communications connection,preferably digital, to the arrangements of gradiometers.

An apparatus embodiment comprises arrays of high order magneticgradiometers located near to or remote from a signal-processing element.The signal-processing element may control and process a singlegradiometer or arrangement or a number of gradiometer arrangements.

In use, a preferred embodiment of the system level aspect of the presentinvention is operated by a central workstation, where an operator canrespond to automatic announcements the system generates. Thisworkstation may be physically attached to the array(s), co-located withthe array(s), or located remotely from the array(s).

In broad terms, a preferred embodiment of the system may be implementedin the form of a plurality of arrangements of gradiometers located at acontrolled location (e.g., airport) having at least one volume ofinterest connected to an operator workstation or workstations. Asignal-processing element is co-located with each of the arrangements ofgradiometers to provide detection, location, or classification of targetobjects.

In broad terms, an embodiment of a signal processing method according toa system level aspect of the invention includes the following steps:

1. The arrangement of gradiometers, when operating, collects data fromeach of the gradiometers by converting analog field gradientmeasurements into a digital measurement utilizing an Analog-to-Digital(A/D) converter. The digital field gradient measurements are sampled ata rate greater than 100 Hz or more preferably greater than 1 kilohertzto maintain the frequency content of the sampled signals. Themeasurement sampling occurs when the gradiometer array has been turnedon to begin a sampling event.

2. The data from each magnetic gradiometer is processed to improveSignal-to-Noise Ratio (SNR) by successively applying afrequency-enhancing filter and then an optional matched signal filter.The matched signal filter may include a filtering window of a magneticdipole moment. The data may also be normalized to provide a uniformnumber of samples for succeeding stages of signal processing.

3. After the SNR enhancement and filtering is performed, the data setfrom a gradiometer is then processed for determination of magneticfeatures. The magnetic features may include magnitude, phase, and timingrelationships, along with frequency spectra or other fundamental signalcharacteristics well known in the art. The collected data represents atime phased scan of an object as it passes by the arrangement ofgradiometers. Conversely, a fixed test volume may be scanned by movingthe arrangement of gradiometers past the volume of interest without anyloss of generality of the technical features disclosed herein.

4. Once each magnetic field reading has been processed at thegradiometer level, each gradiometer in the arrangement presents itsresults and/or raw data to a collection processor. The collectionprocessor may first perform a quality check on the data. At this point,the collection processor utilizes the data from all the gradiometers inthe arrangement to develop an estimate of the locations, magnitudes, anddipole moments of any objects passing a threshold for magnitude. Thecombined data from the arrangement of gradiometers are averaged toprovide a measure of the integrated background. The measured backgroundis subtracted from each gradiometer's raw data so as to leave onlydisturbance data for further processing. The collection processor alsocoordinates timing of the data collection across multiple arrangementsof gradiometers (e.g., multiple portal metal detectors) and provides acommunications node for the arrangements of gradiometers to communicatewith each other or share data or other information (e.g., photographscorresponding to data).

5. The collection processor may utilize all of the gradiometer datacollected to extract further features related to position, magnitudes,or phases of target objects detected.

6. This data set for the collection event is packaged and forwarded to acomputing element dedicated to detection, analysis, or classification ofthe target object.

7. The signature data collected may be processed through a plurality ofindependent processing techniques. A preferred embodiment utilizesmatched filtering, wavelet decomposition, and soft polynomial decisionspace boundaries. The results of the individual classifications may beforwarded to a meta-classifier, where, in one embodiment, a polynomialBayesian or other suitable classifier combines the results to determinea “best” estimate of the target object's position and classification.Other meta-classifiers, such as artificial neural networks, may be usedfor the final classification step.

8. Once the target object is classified, the results are returned to theoperator and optionally transmitted via wire or wireless network fornotifying other systems or personnel. The results may be graphicallydisplayed on an operator interface, or otherwise announced to theoperator for initiating action. Additionally, the results may be used tocontrol the state of the array of gradiometers to prevent furtherscreening from taking place unless the event is deemed benign.

FIG. 2 is a network diagram of a magnetic screening system network 200that includes a magnetic screening system subnetwork 255 and amanagement system subnetwork 260. The magnetic screening systemsubnetwork 255 includes multiple clusters 50 of magnetic screeningsystems 100. In one embodiment, the management system subnetwork 260includes a management station 215, signature database server 220, andnetwork time server 225.

The magnetic screening system 100 includes respective arrangements ofgradiometers 230 and an arrangement processor, also referred to hereinas a portal Central Processing Unit (CPU) 240. In the embodiment of FIG.2, the arrangements of gradiometers 230 are configured in framesdefining the portals 105. The gradiometers 230 and arrangement processor240 communicate with each other via an intra-portal communications bus235. Any standard or application-specific communications protocol may beused for communications over the intra-portal communications bus 235.

The portal CPUs 240 can communicate with one another and the operatorstation(s) 110 via an inter-portal communications bus 245. The screeningcomputers 115 communicate with the portal CPUs 240 and operatorstation(s) 110 via the inter-portal communications bus 245. It should beunderstood that one or more inter-portal communications buses 245 may beused with one or more communications protocols in any combination.Further, the inter-portal communications buses 245 may be wire,wireless, or fiber optic with associated network interface supporthardware and software.

A network device 205, such as a router, may also be connected to theinter-portal communications bus 245 within the cluster 50 of magneticscreening systems 100. The network device 205 provides communicationsservices for the operator station(s) 110 or other processor in thecluster 50 to communicate with other clusters 50 in the magneticscreening system subsystem 255 via an inter-cluster bus 247 using anappropriate protocol. The inter-cluster communications bus 247 may bewired, wireless, or optical.

The magnetic screening system subnetwork 255 may communicate with themanagement system subnetwork 260 via a wide area network (WAN) 210, suchas the Internet. On each side of the WAN 210 is a network device 205that communicates over a WAN communications bus 250. The WANcommunications bus 250 may use an Internet Protocol (IP) communicationsprotocol, Voice-Over-IP (VoIP) communications protocol, or any othersuitable protocol for communicating information or measurement databetween the magnetic screening system clusters 50 and the computingdevices 215, 220, or 225 in the management system subnetwork 260 over amanagement system bus 249. It should be understood that the WAN 210 mayinclude packet switched or circuit switched networks using associatedcommunications protocols.

Within the management system subnetwork 260, the management station 215provides many services. First, the management station 215 provideshigh-level communications with the operator stations 110. Second, themanagement station 215 may provide high level processing, such asanalysis on information or data captured by one or more magneticscreening systems 100. Third, the management station 215 maintains acentral database (not shown) of signatures of target objects measured ina controlled environment or measured by a magnetic screening system 100in the field. The signatures may be maintained in the signature databaseserver 220 or other server(s). Fourth, the management station 215 mayassist in uploading and downloading database of target object signaturesfrom and to the operator stations 110. The uploading and downloadingprocesses allows the magnetic screening systems 100 to share signaturemeasurements of target objects stored in their local databases (notshown) and use all the signatures in the central database stored in thesignature database server 220 for field measurements.

The network time server 225 provides a means of synchronizing andtagging screening events in a uniform fashion across all the elements ofthe system. The preferred embodiment of the system provides for thecommon time base across all elements to produce more value in therecords of incidents and signatures gathered during operation. Thenetwork time server is not required for the system to operate in otherembodiments.

FIG. 3 is a block diagram of the magnetic screening system network 200of FIG. 2 with indications of signals on the data buses. Starting at thegradiometers 230, the intra-portal communications bus 235 carries sensordata 315 from the gradiometers 230 to the portal CPU 240. A portalcamera 305 may be employed at a magnetic screening system 100 andtransmit camera images 310 to the portal CPU 240. The camera images 310may be images of people passing through the respective portal metaldetector 105. The portal CPU 240 may associate the camera images 310with respective target object signatures measured for use in lateridentification of persons carrying a suspected dangerous target object.

The portal CPU 240 sends the sensor data 315, possibly in a processedform 315′, via the inter-portal communications bus 245 to the screeningcomputer 115 optionally with the camera images 310. The screeningcomputer 115 forwards screening results 320 to the operator workstation110 for providing an indication of whether or not the person passingthrough the portal 105 of the magnetic screening system 100 is carryinga target object. The screening computer 115 may provide the screeningresults 320 in the form of signature data, identity of the target object(e.g., gun, knife, cell phone, keys), or classification of the targetobject (e.g., dangerous object, non-dangerous object, unknown object).

In this embodiment, the screening computer 115 also communicates withthe management station 215 via the network paths 245, 247, 249, 250described above. On the network paths, the screening computer 115 andmanagement station 215 communicate queries and responses 325 andoptionally other information or data. The screening computer 115 maycommunicate alarm files 330 and related information or data with thesignature data server 220. The screening computer also communicates timeservice 335 with the network time server 225.

It should be understood that the embodiments of FIGS. 2 and 3 are merelyexamples of possible configurations of processing associated witharrangements of gradiometers 230. For example, the portal CPU 240,screening computer 115, and operator workstation 110 may be combinedinto a single computer system. Similarly, the management station 215,signature data server 220, and network time server 225 may also becombined into a single computer system. In addition, the portal 105 inFIGS. 1 and 2 include gradiometers 230 arranged in two vertical columnsor arrays.

It should be understood that the gradiometers 230 may be configured inany other arrangement(s) that defines at least one boundary of a volumeof interest, e.g., a pathway through which a person walks to be screenedfor target objects. For example, an arrangement of gradiometers 230 maydefine one boundary of a volume of interest and a wall or fixture candefine a second boundary of the volume of interest, where the first andsecond boundaries define a pathway. For example, in one embodiment, onevertical column of the portal 105 is populated with gradiometers 230. Inanother embodiment, a single gradiometer may define a boundary and in amanner similar to other arrangements of gradiometers 230 may be used totake measurements.

FIG. 4A is a block diagram of an individual gradiometer 230. In oneembodiment, the gradiometer 230 includes a processor 410, such as adigital signal processor (DSP), to communicate with at least threemagnetometers 400 a, 400 b, and 400 c (collectively 400) using anintra-gradiometer bus 230. The gradiometer DSP 410 also communicateswith the arrangement processor 240 over the intra-portal bus 235.

The processor 410 may be analog, substantially digital, or completelydigital depending on various factors for design implementation. Itshould be understood that supporting circuitry (not shown) which allowsthe gradiometer processor 410 to communicate with the magnetometers 400,may also be employed. Examples of other circuitry include memory,registers, analog circuits, or supporting processors. Further, thegradiometer processor 410 may include multiple gradiometer processors410 for parallel processing purposes.

In addition to a uniaxial layout of the magnetometers 400, themagnetometers may also be positioned offset in one or more axes fromeach other for purposes of achieving particular orientations ofsensitivity for the gradiometer 230.

In other embodiments, the gradiometers 230 in an arrangement may nothave an “on-board” processor 410. In such a case, the arrangementprocessor 240 performs functions described herein in reference to thegradiometer processor 410.

FIG. 4B is a electrical schematic diagram of an example embodiment ofthe gradiometer 230. The gradiometer processor 410 communicates with afield programmable gate array (FPGA) 412, which, in turn, communicateswith the magnetometers 400.

Each magnetometer 400 in this embodiment includes the same circuitry, sothe following description applies to each of the magnetometers 400. Atthe front end of the magnetometers 400 is a digital-to-analog converter(DAC) 415. In one embodiment, the DAC 415 includes a left (L) outputchannel 417L and a right (R) output channel 417R (collectively 417).Both of the output channels 417 connect to an input of a magnetic sensor420, which senses a gradient in a magnetic field. The output of themagnetic sensor 420 is connected to an amplifier 425, such as a senseamplifier capable of amplifying very low level voltages without addingsignificant noise in the amplifying process. The output of the amplifier425 is an analog-to-digital converter (ADC) 430. The ADC 430 provides adigital output to the FPGA 412, which, in turn, provides a digitalsignal to the gradiometer processor 410.

In operation, the gradiometer processor 410 issues a command signal tothe FPGA 412 for commanding one or more of the magnetometers 400. Thecommand signals output by the processor 410 may correspond to a mode ofoperation of the gradiometer 230, including measurement mode, backgroundoffset reduction mode, calibration mode, self-test mode, automaticalignment mode, diagnostic mode, or tracking mode. These modes aredescribed in detail below beginning in reference to Table I and FIG. 9.

The FPGA 412 transmits the command received from the processor 410 tothe corresponding magnetometer(s) 400. As illustrated, an offset controlsignal 435 output by the left channel 417L of the DAC 415 indicates thatthe processor 410 is commanding the magnetometers 400 to operate atleast part time in background offset reduction mode. As alsoillustrated, a bridge drive signal 440 output by the right channel 417Rof the DAC 415 indicates that the processor 410 is commanding themagnetometers 400 to operate at least part time in measurement mode oranother mode that causes the magnetic sensor 420 to take measurements.For example, both signals 435 and 440 are used during background offsetreduction mode since one portion of the mode is a measurement period(bridge drive signal 440) of a background disturbance causing a magneticoffset of the magnetometer 400, and another portion of the backgroundoffset reduction mode is an offset reduction period (offset controlsignal 435). Further discussion of the background offset reduction modeand other modes is presented below in reference to FIGS. 9 through 12C.

FIG. 5 is a detailed electrical schematic diagram of the magnetometers400 of FIG. 4. In this embodiment, the magnetic sensor 420 includes amagnetic transducer 500, which is in the form of a traditionalWheatstone bridge. The transducer 500 includes two legs of staticelements 505 a and two legs of variable elements 505 b. In the case of amagnetic transducer, the static elements 505 a and variable elements 505b are the same elements, but a magnetic shielding is placed over thestatic elements so that external magnetic disturbances influences onlyaffect the unshielded variable elements 505 b. The type and arrangementof the variable elements of the magnetometer is not material to theoperation of the gradiometer. In other embodiments, the magnetometerbridge may have one or multiple active sensing elements. Such otherarrangements are well known to practitioners of the art inmagnetometers.

In operation, the DAC 415 presents the bridge drive signal 440 to abridge driver amplifier 515, which may produce a bridge drive current520 whose level is set, in part, by a compensation resistor 510providing a voltage VRC at a junction with the bridge driver 515negative input. The bridge driver 515 can then correct error in thebridge drive current 520. Thus, the transducer 500 produces adifferential voltage output Vb 525 that is amplified by the amplifier425 whose output is sampled by the ADC 430.

The offset control signal 435 is presented to an offset control circuit535, which includes a drive amplifier 540 and magnetic field generator540 for producing a magnetic field that drives offset of the magnetictransducer 500 to a “reduced” state. The reduced state is a state inwhich background offset caused by large ferromagnetic elements in thevolume of interest or within a zone of influence of the gradiometer iscancelled from the magnetic transducer 500 (i.e., background offsetreduction mode).

It should be understood that the magnetic transducer 500 may be otherforms of magnetic transducers known in the art adapted to detectmagnetic fields as described herein. For example, the magnetic field maybe the earth's magnetic field (˜45,000 nTeslas) and fields of targetobjects (100 nTeslas or less). The magnetic fields may also be muchlarger, as in the case of medical sensing applications. Therefore, theoffset control circuit 535 is preferably capable of producing a magneticfield over a wide range or the offset control circuit 535 is speciallydesigned for particular sets of applications.

FIG. 6A is an electrical schematic diagram of a calibration circuit 600that may be used to calibrate the magnetic sensors 420 in themagnetometers 400. The calibration circuit 600 includes a precisioncurrent source 605 with feedback circuit 607 to produce a precisioncalibration drive current 608. The precision calibration drive current608 travels on a pair of crossing traces 610 to a magnetic fieldgenerator 615, which may be a simple wire loop or more complex magneticfield generator 615. The magnetic field generator 615 produces aprecise, known, calibration magnetic field 617 in magnetic relationshipwith the magnetic sensor 420 of the magnetometer 400 and causes ameasurable response by the magnetometer 400. The output Vb 525 of themagnetic sensor 420 may be fed to a controller 625, which drives anoffset correction drive circuit 635 and magnetic field generator 640 forproducing an offset correction magnetic field 645 for correcting theoffset caused by the calibration circuit 600.

The offset correction circuitry 635, 640 may be the offset controlcircuit 535 used for calibration in this case, or may be a separatecircuit. In either case, any number of signals used to correct for themeasured response of the magnetometer 400 can be used as a calibrationmetric, which may be scaled or offset in real-time or post-processing.

The controller 625 may be an analog controller or a digital controller.In the case of a digital controller, it may be implemented in thegradiometer processor 410 or in a separate digital processor. In eitherof the digital processor cases, the magnetic sensor output Vb 525 is asampled form provided by the ADC 430 that is sampled according totechniques well known in the art. Further, the controller 625 may useany applicable control law, such as a proportional, integral,differential (PID) control law. Since digital controllers can be updatedin software, the controller 625 is preferably a digital controller.

Continuing to refer to FIG. 6A, a calibration cycle includesmeasurements of at least three calibration points in one embodiment. Atiming sequence is listed at the input to the calibration precisioncurrent source 605. A first calibration point (Cal A) does not use thecurrent source 605 to produce a drive current 608. Instead, the firstcalibration point is a measure of the magnetometer 400 immediately afterbackground offset reduction mode has removed offset from themagnetometers 400 and calibration measurements of Vb 525 are taken andaveraged over 100 msec, for example. A second calibration point (Cal B)applies a low level voltage to the current source 605 to produce a lowlevel drive current 608 for 100 msec, for example, during whichmeasurements are taken and averaged. A third calibration point (Cal C)applies a high level voltage to the current source 605 to produce a highlevel drive current 608 for 100 msec, for example, during whichmeasurements are taken and averaged.

FIG. 6B is a plot 650 of results of the calibration measurements basedon the example calibration cycle just described. The plot includes threepoints, 655 a, 655 b, and 655 c corresponding to calibration points, CalA, Cal B, and Cal C. A linear fit, polynomial fit, or other suitabletechnique may be employed to determine a calibration curve 660. Thegradiometer processor 410 uses the calibration curve 660 to improveaccuracy of measurements of gradient magnetic fields by themagnetometers 400 in a manner well known in the art.

At the gradiometer design level, the principles of the present inventionprovide a method or corresponding apparatus to arrange and processsignals from an array of multiple magnetic sensing devices (e.g.,gradiometers) to control near and far field responses of thearrangement. The arrangement may then be used to sense signals (e.g.,local disturbances in the earth's magnetic field), which are near themagnetic sensing devices in a volume of interest, while excludingsignals that are in a particular direction away from the volume ofinterest.

One advantage of this aspect of the present invention is that the userof the magnetic sensing device does not need to shield or otherwiseprevent extraneous signals from interfering in the measurement of thedesired signals or target objects. By improving the directionality ofthe magnetic sensing device, expense and complexity of otherwiseeliminating external signals or sources of interference is saved.

Another advantage of this aspect of the present invention is that theimproved directionality of the magnetic sensing device allows the usageof the magnetic sensing device in an area otherwise unsuitable formagnetic measurements. This is accomplished by automatically excludingbackground signals normally encountered during measurement, such assignals from light fixtures, power lines, and ferromagnetic objects inclose proximity to the magnetic sensing device.

The principles of this aspect of the present invention may beimplemented in the form of a plurality of magnetic sensors, such asvector magnetometers, to form a gradiometer. A preferred embodiment ofthe gradiometer does not require a specific number of magnetometers or aspecific technology of magnetometers. Using the technique describedherein can optimize the gradiometers.

The magnetometers may be Superconductivity Quantum Interfering Devices(SQUID), Anisotropic Magnetoresistive (AMR) or Giant Magnetoresistive(GMR) sensors, spin tunneling devices, or simply a wire loop or solenoidfor the magnetic field detector.

Once a gradiometer has been devised, the magnetometers can be optimizedby application of a technique for determining distance and weighting ofthe magnetometers. The distance between the outermost magnetometer pairis selected based on the application in which the gradiometer is to beused. For example, compact baselines may be preferred for “close-in”medical magnetic field measurements, while large baselines may bepreferred for large-scale field measurements, such as the location oflarge ferromagnetic objects or deposits on the ocean floor. Thetechnique of optimization does not require a specific baseline forsuccessful application.

The development of a unidirectional gradiometer entails positioning atleast one magnetometer between the outermost magnetometers on a selectedbaseline. One common configuration in the art is called a second ordergradiometer, discussed below in reference to FIGS. 7C and 7D. The secondorder gradiometer rejects only the signals along the perpendicular fromthe baseline between the two magnetometers.

A digital signal processor or other digital computing device may beemployed to operate a preferred embodiment of the gradiometer. Analogprocessors are also possible once the magnetometers' positions andweights have been set; however, an analog processor has a drawback inthat it is not easily adjustable once fabricated. The proposed digitalgradiometer has the advantage that the pattern of sensitivity can beadjusted as required after the fabrication through digitally updatingthe weights associated with the respective magnetometers that are storedin the digital processor or memory associated therewith.

In broad terms, a preferred embodiment of the gradiometer includes atleast three magnetic sensors in linear alignment on a common axis. Asensing axis and polarity of each magnetometer is aligned with that ofthe other magnetometers along the common axis. The positions of theoutermost magnetometers are selected based on criteria for theapplication of use. The position of the magnetometer(s) between theoutermost magnetometers is/are calculated utilizing an embodiment of amagnetometer positioning/weighting optimization method.

In broad terms, an embodiment of the magnetometer positioning/weightingmethod includes the following steps:

1. Once a baseline has been selected, the number of “inner”magnetometers is chosen depending upon the user's desire for directivityor other technical criteria.

2. The position of the magnetometers is established using any applicablenumerical method.

3. The position and vector magnitude of the interfering and desiredmagnetic signals are modeled, optionally utilizing the same numericalmethod as in Step 2 immediately above.

4. An equation for received magnetic signal combinations is input to anumerical optimizer, such as least squares or successive approximationoptimizer, as is common in the art.

5. Constraints of the method are entered into the optimization equation.The constraints include:

(a) The sum of the signal weightings must be zero.

(b) The received interference signal must be zero or reduced to adesired value.

(c) The received signal of interest must be maximized.

6. The optimizer then changes the signal weighting while moving theinner magnetometers until a satisfactory solution is found. The valuesof the weightings are not constrained to positive-only or negative-onlyvalues. The values can be continuous or discrete, positive or negative,so long as the conditions of Step 5 above are met.

7. Once the numerical optimizer has reached a level of beingsufficiently close to the signal goals, the optimization process can bestopped, and the calculated values can be used for construction of themagnetic field gradiometer(s).

FIG. 7A is a diagram of a gradiometer 230 with magnetometers 400 a, 400b, and 400 c positioned using the process just described. In oneembodiment, the two outer magnetometers 400 a and 400 c are arbitrarilypositioned, and at least one other gradiometer 400 b is positionedrelative to the outer two magnetometers 400 a and 400 c. Unequal weightsare calculated for the magnetometers that the processor 410 uses toscale outputs of the magnetometers 400. The processor 410 combines thescaled outputs to orient a direction of gradiometer sensitivity,represented by a sensitivity lobe 700 a, toward a volume of interest,which is to the left in the example of FIG. 7A.

In one embodiment, the positions of the magnetometers 400 aredetermined, optionally along a single axis, according to a number ofparameters, such as available size of a structure into which thegradiometer 230 is to be deployed. To improve sensitivity of thegradiometer, the outer two magnetometers 400 a and 400 c are preferablypositioned as far apart as possible within a given constraint. Themiddle magnetometer 400 b in a portal metal detector application ispositioned closer to the outer magnetometer 400 c farther away from thevolume of interest without touching that outer magnetometer 400 c.

In this embodiment, after the positions of the magnetometers 400 areset, the weights are calculated preferably using deterministicmathematical techniques, such a through use of a least squaresoptimization technique. For flexibility of design, the weights can benon-integer weights. Use of non-integer weights allows the sensitivitylobe 700 a to be optimized for use in a given application. In the caseof the gradiometer 230 employing a digital processor 410, the digitalprocessor 410 can adjust the weights during operation, thereby changingthe characteristics of the sensitivity lobe 700 a. Adjusting the weightsto change characteristics of a sensitivity lobe can be compared tochanging phase delays in a phased array radar system to effect thesteering of the sensitive lobe in a desired direction.

FIG. 7B is a detailed plot of the sensitivity lobe 700 a. The positionsof the magnetometers 400 a, 400 b, and 400 c are represented by lowercase ‘x’ in the plot. As expected, sensitivity is much higher closer tothe magnetometers 400 than farther away from the magnetometers. Ofparticular note, the sensitivity lobe 700 a does not extend to the rightof the magnetometers 400 based on the spacing and weighting.

FIG. 7C is a diagram of the gradiometer 230 in which the magnetometers400 are positioned equally distributed along a single axis and havingequal weights assigned thereto for scaling their outputs. Acorresponding sensitivity curve 700 b, which is peanut-shaped, indicatesthat the sensitivity of the gradiometer 230 extends into a volume ofinterest (to the left) and also into a volume that may not be ofinterest (to the right). As a result, a magnetic disturbance that isoutside the volume of interest, such as a wall, other portal metaldetector, or other machine, can influence measurement results in adetrimental manner.

FIG. 7D is a detailed plot of the sensitivity lobe 700 b correspondingto the equally-spaced, equally-weighted embodiment of FIG. 7C.

FIG. 8 is a vector diagram which, along with the equations that follow,provides a more detailed analysis of the process used to make and usethe gradiometer 230 of FIG. 7A.

The magnetic field produced by a single magnetic dipole is given by:$\begin{matrix}{{B\left( r_{i} \right)} = {\frac{\mu_{0}}{4\pi}\left( {\frac{3\left\lbrack {M \cdot \left( r_{i} \right)} \right\rbrack}{r_{i}^{4}} - \frac{M}{r_{i}^{3}}} \right)}} & {E1}\end{matrix}$

Where r_(i) is the vector position of the dipole 800 relative to thesensitive elements 400 a, 400 b, 400 c making up the preferred secondorder gradiometer. M is the vector dipolar magnetic moment of the targetobject. Complex objects that make up the targets of interest may beviewed as a collection of dipole objects with their magnetic fieldssuperimposed upon each other without any loss of generality. Theoptimization method holds for complex objects as well as simple objectsin this formulation.

The sensed magnetic field at each magnetometer is proportional to thearea, A, of the magnetometer and the orientation of the sensitive axis,V, to the dipole moment's principal axis. The responsitivity constant εof the sensor material in units of changed characteristics (resistance,voltage, current) per unit area is also included to properly scale theexpected output of the magnetometer. The response of the individualmagnetometer is then:Q _(i) =ε·A·B(r _(i))•V _(i)  E2

The equation indicates the measured signal is proportional to thecollection area multiplied by the sensitivity of the materials used andthe vector product of the field strength and direction of the targetobject relative to the sensitive axis of the magnetometer.

The preferred gradiometer configuration 230 includes threemagnetometers, each having a different response to the target object dueto differing geometries and aspects relative to each other and thetarget object. The summary response of the k-th individual gradiometeris made up of the weighted sum of the individual responses of the nmagnetometers: $\begin{matrix}{T_{k} = {\sum\limits_{i = 1}^{n}{a_{i} \cdot Q_{i}}}} & {E3}\end{matrix}$

T_(k) is the output of the magnetic field gradient sensed by thegradiometer 230. The same equation is used as the basis for theoptimization of the a_(i) coefficients allowing the practitioner toadjust the response lobes of the gradiometer as described elsewhere inthis document.

Turning now to operation of the gradiometers 230 and arrangement ofgradiometers 230, the many processors associated with the gradiometers230 cause them to operate in multiple modes.

At the multi-modal operational level, the present invention includes animplementation of a gradiometer having at least three vectormagnetometers. In a preferred embodiment, the magnetometers are eachindependently controlled and measured. Each magnetometer may includeindependent biasing, control, and measurement circuits. Offset of themagnetometers is controlled without use of an additional magnetometerspecifically designed to do so. Common mode coupling is eliminated byindependence of the individual magnetometers in preferred embodiments.The effect of the independence allows adjustment and refinement of theoverall gradiometer output without undesirable effects of convolving theerrors of the magnetometers together.

The magnetometers within the gradiometer can be switched between anactive measurement mode and a non-active mode, where the non-active modeis also referred to herein as a “background offset reduction mode.”During the non-active mode, correction for a plurality of errors commonto vector magnetometers can be accomplished by the gradiometer orprocessor(s) associated therewith.

One advantage of the multi-modal aspect of the present invention is thatthe gradiometer performance can be continuously maintained against driftand errors induced by changing environmental conditions. Anotheradvantage of the multi-modal aspect of the present invention is that,because of the continuous adjustments against drift and errors, thegradiometers can produce data at high rates. Common gradiometers producedata at 10 to 20 Hz rates. The gradiometer of the present invention canproduce rates up to 50,000 Hz with no loss of dynamic range or accuracy.The gradiometer 230 at its core comprises at least three vectormagnetometers 400. Each magnetometer 400 may have a bias source drivenby a controlled Digital-to-Analog (D/A) converter, an Analog-to-Digital(A/D) converter for the measured magnetic field, anddigitally-controlled support circuitry for offset adjustment, typicallyby another, independent, digital-to-analog conversion. In use, apreferred embodiment of the multi-modal gradiometer is operated by aDigital Signal Processor (DSP) element that can process the data fromthe magnetometers, provide offset estimations, and perform otherfunctions for successful operation of the circuit.

Table I provides a listing of the multiple modes and a definitioncorresponding to each: TABLE I Mode Definition Measurement mode Use ofgradiometers 230 to measure a volume of interest for target objects.Background offset Use of gradiometers 230 to measure a reduction modebackground offset and processing applied in either real-time orpost-processing to remove the background offset from measurements.Calibration mode The processor 410 applies a magnetic field generatedlocally at the magnetometers 400 and measures a reaction by themagnetometers 400 to determine a calibration curve for compensatingmeasurements made by magnetometers 400. Self-test mode The processor 410puts components of the gradiometer in states to compare measuredperformance of the components in those states against specifiedperformance in those states to determine operational readiness.Automatic The processor 410 captures and averages magnetic alignmentmode field strength over a long duration while compensating forbackground disturbances to calculate the alignment of the gradiometers230 relative to the earth's magnetic field or other magnetic fieldproviding a common influence on the gradiometers 230. Relativeorientations of gradiometers 230 can be determined and used duringmeasurements. Diagnostic mode The process 410 captures and outputs themeasured field strengths by each magnetometer in an unaltered state(i.e., raw measurement data).

FIG. 9 is a timeline 900 of an example of multiple modes of operation ofan arrangement of gradiometers 230. The timeline 900 includesalternating modes of operation: background offset reduction mode 905 andcalibration mode 910/measurement mode 915. The timeline 900 can beapplied in at least two different ways for using the gradiometers 230 ina portal metal detector 100 application. The first way the timeline 900can be applied is to alternate modes of operation while processing aline of people proceeding through the portal 105 of the magneticscreening systems 100 of FIG. 1, where background offset reduction mode905 occurs at times no one is walking through the volume of interest andcalibration mode 910 and measurement mode 915 occur at times someone iswalking through the volume of interest. The timeline 900 can also applyto a time when someone is walking through the volume of interest, wherethe modes 905, 910, and 915 alternate a selectable number of times. Themore times the background offset is reduced and calibration isperformed, the better the accuracy of the measurements.

FIG. 10 is a timing diagram 1000 showing relative timing of portions ofthe timeline 900 and the bridge bias voltage 440′. In this embodiment,during opposite periods of a 1 msec frame time 1020 or other ratesuitable for the application in which the gradiometers 230 are employed,a process executing the measurement mode of operation switches betweencalibration mode 910 and measurement mode 915. Alternatively, theprocess may include background offset reduction mode 905, which includesset/reset settle 1010 a and active background control 1010 b followed bymeasurement mode 915. In another embodiment, calibration mode 910 andbackground offset reduction mode 905 are executed in a selectable manneron opposite phases of the bridge bias voltage 440′ from measurement mode915. For example, a timing sequence may be as follows: background offsetreduction mode 905, measurement mode 915, calibration mode 910,measurement mode 915, and repeat. In another example, background offsetreduction mode 905 may not occur during the same phases. In yet anotherexample, background offset reduction mode 905 occurs every nth time ameasurement mode 915 occurs. In still yet another embodiment,calibration mode 910 occurs every nth time a measurement mode 915occurs. It should be understood that any number of combinations of modesequences can be employed, which may be dictated by a false alarm rateor other metric associated with the measurements.

Continuing to refer to FIG. 10, calibration mode 910 includes measuringpositive samples 1005 a and negative measurements 1005 b correspondingto the bridge bias voltage 410′. Similarly, measurement mode 915includes measuring positive samples 1015 a and negative samples 1015 bcorresponding to the bridge bias voltage 410′. Multiple samples may bemeasured and averaged or otherwise computed to determine a noiseresistant measurement. In addition, the background offset reduction mode905 includes a set/reset settle time 1010 a, during which themagnetometers 400 are driven to reset, and an active background controltime 1010 b, during which a background magnetic field is measured andits effect on the magnetometers 400 is reduced.

FIG. 11A is a graphical diagram of an example application in which anarrangement of gradiometers 230 is deployed to detect target objectscarried by a person. In this example, the person 125 at time T1, walkingfrom left to right, approaches the portal 105 of the magnetic screeningsystem 100. During this time (T1), the magnetic screening system 100operates in the background offset reduction mode 905. As the person 125approaches the portal 105, the person 125 passes a first pair of opticalsensors 1105 a that senses the person 125 disrupting an associatedoptical beam 1110 a. In response, the magnetic screening system 100 incommunication with the first pair of optical sensors 1105 a exitsbackground offset reduction mode and enters measurement and calibrationmodes 910, 915, during time (T2).

During time T2, the person 125 passes through the volume of interest 75whose boundaries are defined on both sides, in this embodiment, by thegradiometers 230 deployed in the vertical columns on either side of theportal 105. The sensitivity lobes 700A, described above in reference toFIGS. 7A and 7B, extend through the volume of interest 75 and enable thegradiometers 230 to detect any ferromagnetic objects being carried bythe person 125. The person 125 continues to a second pair of opticalsensors 1105 b having its own optical beam 1110 b, which is interruptedas the person 125 passes. Upon notification that the person 125 hasexited the volume of interest 75, the magnetic screening system 100again returns to background offset reduction mode 905 in time T3.

It should be understood that the optical sensors 1105 are an example ofsensors that can be used to detect when the person 125 is approaching orleaving the volume of interest 75. Motion detectors, active floor mats,or the magnetic screening system 100 itself may also be used to detectpositions of the person relative to the volume of interest 75. In otherembodiments, the system 100 may be operated without sensors fordetection of entry of a person in the volume of interest 75. Forexample, an operator 130 (FIG. 1) may trigger measurement mode to begin.As another example, measurement mode may occur continuously withcalibration mode and background offset reduction mode being used on aperiodic basis, on an “as needed” basis such as through automatictriggering based on a metric associated with a gradiometer 230 ormagnetometer 400, or on any other basis.

Of particular interest in the example application 1100 of FIG. 11A isthat the person 125 does not need to remove his jacket 1115 to allow themagnetic screening system 100 to detect, identify, or classify any metalobjects being carried therein. In addition, the person 125 does not needto remove from his clothing any potential target objects, such as a cellphone, loose change, keys, or other ferromagnetic items, that can besensed by the magnetic screening system 100 incorporating the principlesof the present invention on some or all of the different levelsdescribed herein.

FIG. 11B is a representation of the timing sequence 1100 illustrated inFIG. 11A. T1, T2 and T3 are shown in relation to multiple timingdiagrams corresponding to the time T2 in which the person 125 is passingthrough the volume of interest 75. As indicated, once the person 125interrupts the first optical beam 1110 a, the magnetic screening system100 begins to take samples and produce output samples 1105. As describedabove in reference to FIG. 10, the bridge bias voltage 440′ occurs with(i) calibration mode 910 and optionally background offset reduction mode905 occurring during a first period and (ii) measurement mode 915occurring during a second period.

At the start of the measurement process, a set/reset pulse 1130 atriggers positive to indicate that the measurement mode 915 has begun.During the measurement mode 915 period of the bridge bias voltage 440′,the ADCs 430 (FIG. 4B) samples at a high rate, such as 10 kHz or higher,in this embodiment, to produce sixteen samples 1120 a during thepositive portion of the bridge bias voltage 440′ and sixteen samples1120 b during the negative portion of the bridge bias voltage 440′. Thesamples are processed by the processor 410 and other processors, such asthe screening computer 15 (FIG. 2). ADC samples 1115 a during the firstperiod of the bridge bias voltage 440′ are considered invalid since theyare taken during a “dead time,” which is the period as described aboveduring which the calibration mode 910 or background offset reductionmode 905 may be occurring and, therefore, the measurements do not relateto any target objects. The ADC samples 1115 b during the measurementmode 915 may be decreased through decimation, for example, an output asoutput samples 1125 from the ADC 430 (FIG. 4B) for processing by theprocessor 410.

The timing diagram 1100 during the measurement period in time T2continues and repeats so long as the person 125 in FIG. 11A is betweenthe first and second pairs of optical detectors 1105 a, 1105 b, i.e., inthe volume of interest 75. If a continuous line of people are passingthrough the volume of interest 75, the measurement mode may continuewithout interruption, and, in such a case, it is preferable that thebackground offset reduction mode 905 occur on a regular basis to ensurefull dynamic range of the magnetometers 400 is maintained so that thegradiometer is maintained in a linear operating region.

It should be understood that the example timing diagram of FIG. 11B maybe different in alternative embodiments of the arrangement ofgradiometers 230. It should also be understood that the time frame 1020of 1 msec may be faster or slower in such other embodiments. Forexample, the gradiometer processors 410 may cause the gradiometers 230to sample at a rate greater than 50 Hz. The gradiometer processors 410may also cause the gradiometers to switch between the measurement modeand the calibration mode at a rate greater than 0.1 Hz. In such anembodiment, the portal metal detector system 100 calibrates very slowly,as might be the case during a slowly passing person 125. In otherembodiments, it is preferable to enter calibration mode 910 at least afew times while the person 125 is in the volume of interest 75. Thus,although calibrating at a 1 kHz rate may be excessive in someapplications, it provides for a more accurate measurement. However, itshould be understood that overall system error budgets can be achievedwith slower rates, so the rates can be determined depending on theapplication or on a case-by-case basis.

FIGS. 12A-12C are system-level diagrams in which the assembly ofgradiometers 230 can be used to detect target objects, such as a gun1250 a. In each of these embodiments, the measurement mode, backgroundoffset reduction mode, calibration mode, self-test mode, automaticalignment mode, diagnostic mode, or tracking mode may be employed.

Beginning with FIG. 12A, the target object 1205 a, which produces aparticular magnetic field disturbance 1210 a due to the target object'sinfluence on the earth's magnetic field or other magnetic field,traverses on a path 1215 on a person 125 into a volume of interest 75 inthe portal 105. The gradiometers 230 in the portal 105 are operating ina measurement mode, and, as described above, their sensitivity lobes 700a are directed horizontally into the volume of interest 75 throughproper design, as described above.

FIG. 12B illustrates the portal 105 with gradiometers 230 operating inthe tracking mode. In the tracking mode, pluralities of the gradiometers230 generate real-time tracks of target objects in three dimensions.Thus, instead of only providing horizontal look angle data by pairs ofgradiometers 230, as illustrated in FIG. 12A, the gradiometers 230 ofFIG. 12B sense and report three-dimensional positional information ofthe target object 1250 a. The portal CPU 240 and screening computer 115(FIG. 2) can provide the processing power for operating the gradiometers230 in the tracking mode.

FIG. 12C is a top view of an alternative embodiment of a magneticscreening system 100 in which an arrangement of gradiometers 230 isemployed to provide three-dimensional tracking information. In thisembodiment, at least three gradiometers 230 are distributed about avolume of interest 75 for detecting, identifying, classifying, tracking,or combination thereof, target objects, such as a gun 1205 a or cellphone 1205 b. The cell phone 1205 b has an induced magnetic fielddisturbance 1210 b that is different from the magnetic field disturbance1210 a produced by the gun 1205 a in the earth's magnetic field or othermagnetic field. In either case, the arrangements of gradiometers 230 maybe distributed in non-portal like fixtures, such as wastebaskets,planters, vending machines, or other discreet security fixtures, so asnot to be intrusive or noticeable by patrons of a venue, such as anamusement park, sports arena, airport, government building, or otherplace in which detection of ferromagnetic objects is of interest.

In addition to providing a tracking mode, the processors associated withthe arrangements of gradiometers 230 can initiate an auto-alignmentprocess for each gradiometer. The arrangement processor 240 (FIG. 2) incommunication with its respective arrangement of gradiometers 230 causesthe gradiometers 230 to capture and average magnetic field strength overa long duration while compensating for background disturbances tocalculate the alignment of the gradiometers 230 relative to the earth'smagnetic field. In this way, the processors, such as the arrangementprocessor 240, screening computer 115, or other processor used forauto-alignment, can determine relative orientations of each gradiometer230 to at least one other gradiometer in the system. Thus, in thescenario 1200 a of FIG. 12A, it can be seen that the gradiometers 230directly across from one another in the portal 105 are aligned withrespect to one another. In the portal 105 used in the scenario 1200 b ofFIG. 12B, it can be seen that all gradiometers 230 are known relative toall other gradiometers 230 in the portal 105. In the scenario 1200 cdepicted in FIG. 12C, multiple arrangements of gradiometers 230 knoworientations of other arrangements of gradiometers 230. In this way,tracking target objects 1205 a, 1205 b can be done with little set-uptime and added expense.

In some embodiments, a diagnostic mode is possible in which theprocessor 410 (FIG. 4A) associated with the magnetometers 400 cancapture and output the measured field strengths by each magnetometer 400in an unaltered state. A higher-order processor such as the portalprocessor 240, screening computer 115, operator workstation 110,management station 215, or other processor tasked to run diagnostictests can determine whether a failure, error, or other impairment tonormal operations of any gradiometer 230 in the magnetic screeningsystem 100 or magnetic screening system network 200 (FIG. 2) is apotential to adversely affect measurement.

FIG. 13 is a time plot of a signature representing a measurement of atarget object, such as a gun 1205 a, passing through a volume ofinterest 75 as measured by gradiometers 230 in a portal 105 or otherarrangement of gradiometers 230. The signature 1300 is therefore arepresentation of a time measurement of a target object's magnetic fieldinduced by the earth's magnetic field as measured by the gradiometers230 as a person 125 carries the target object through a volume ofinterest 75 in which the gradiometers 230 have oriented their directionsof sensitivities. As well understood in the art, the signature 1300 isdifferent for every target object. Therefore, the signature 1300 for agun 1205 is different for the signature for a cell phone 1205 b, soprocessors 410, 240, or other processors adapted to identify or classifythe target objects can do so if (i) the resolution of measurement ishigh enough (i.e., the number of samples taken as a target object passesthrough a volume of interest is at a high enough rate) and (ii) theprocessing is adapted to discern differences between or among targetobjects or classifications of target objects.

To determine signatures for target objects, there are several ways totrain a magnetic screening system 100. One way to train the system 100is to perform a measurement of a target object in multiple orientationsas it progresses through a volume of interest 75 being measured by anarrangement of gradiometers 230. Such a case can be done in a controlledmanner with a robot moving a target object through the volume ofinterest 75 in different orientations. The signature 1300 is capturedand stored in a database along with an identifier and optional graphicalrepresentation of the target object for use in field deployed magneticscreening systems 100.

Another way to train the magnetic screening systems 100 is for a targetobject unknown to the system to cause an alert to an operator of themagnetic screening system 100 that the target object is unidentified(i.e., its signature is not found in a local signature databasemaintained by the magnetic screening system 100, cluster 50 of magneticscreening systems 100, or network wide level. In such case, the operatorof the magnetic screening system 100 can visually inspect the targetobject, which may be, for example, a newly-released cell phone or otherferromagnetic object newly introduced in the consumer market, forexample. The operator of the system can then add the signature of thetarget object to a database at the arrangement of gradiometers level,cluster of arrangements of gradiometer level, or network wide level, andassociate a description, identifier, or graphical representation withthe signature in the database. In this way, the next time the samesignature is identified by a magnetic screening system 100 having thesignature in its database, the system 100 can inform the operator ofwhat the target object is. If the same target object is carried througha volume of interest 75 and again not recognized by the magneticscreening system 100, it may be because the target object was carriedthrough the volume of interest in a different orientation from theprevious time in which the identity of the target object was determinedand entered into the signature database(s). In this case, the operatorof the magnetic screening system 100 can choose to enter the newsignature to the database and associate it with the identifier,classification, or graphical representation previously entered in thesignature database. In this way, the magnetic screening systems 100 canadaptively learn of new target objects without having to be learned in acontrolled environment by a manufacturer, distributor, or other companyassociated with producing, distributing, or selling magnetic screeningsystems 100. In some embodiments, signatures measured or previouslyassociated with known target objects are displayed on the operatorstation 110 to allow the operator 130 to make an informed choice as towhether to add the newly acquired signature 1300 to the database(s).

In the magnetic screening system network 200 of FIG. 2, the screeningcomputers 115 may transmit respective local databases of signatures tothe management station 215 for storage of the newly identifiedsignatures 1300 corresponding to the target objects periodically or onan event driven basis. For example, periodically may mean the localdatabase of target object signatures is uploaded to the signaturedatabase server 220 on an hourly basis, daily basis, weekly basis, ormonthly basis. An event driven basis may be done as a result ofdetection of a target object, either known or unknown, an operatorrequest, or upon initiation of a self-test.

The management station 215 may also transmit the central database ofsignatures stored on the signature database server 220 to the localdatabases stored at the magnetic screening systems 100, for example,periodically or on an event driven basis. The event driven basis in thiscase may be on receipt on an unknown target object from one of thescreening computers 115, an operator request, receipt of a newsignature, a system reboot, or a system power-up. It should beunderstood that other events, foreseen or unforeseen, may also be usedas a trigger to either upload or download signatures of target objects,either previously known or unknown, for use by the magnetic screeningsystems 100 to continually improve on their ability to detect, identify,classify, or otherwise recognize a target object so as to continue toreduce a rate of false alarms, which ultimately result sin higher speedprocessing of people passing through the volume(s) of interest 75.

FIG. 14A is a time plot 1400 a of a measurement of the target signature1300 as it passes through the volume of interest 75. In this example, asource of background offset affects measurements by the gradiometers230. The effect manifests itself in the form of a slope of the curveover which the target object's signature 1300 is superimposed. Atrepeating intervals, the magnetometers 400 in the gradiometer 230 arereset, as described above, so as to ensure full dynamic range for thenext measurement. A source of background offset is a wheelchair, forexample, that is nearby the arrangement of gradiometers 230. The periodfor reset may be when the person 125 crosses through the optical beam110 a (FIG. 11A) or may be on a sample-by-sample basis at a 1 kHzinterval (FIG. 11B) or other measurement rate.

FIG. 14B is a plot 1400 b of the measurement curve 1400 a of FIG. 14Awith the slope of the curve removed through use of a background offsetreduction process, either in real-time in a sample-by-sample basis orthrough use of post-processing. In the case of post-processing, insteadof resetting the magnetometers 400 before each sample measurement ismade, a measurement of the background offset is captured and associatedwith each sample point and provided to the arrangement processor 240 foruse in mathematical removal of the background offset. The result is acurve 1400 b with offset removed so that the target signature 1300 ismore easily discernable. In addition, the target signature 1300 can benormalized, optionally in amplitude, time, phase, or combinationthereof, for more easily being matched to a signature in a localsignature database or central signature database. It should beunderstood that various processing improvements may also be applied. Forexample, after normalization, a data reduction process, encryptionprocess, time-frequency analysis, or other processing may be employed soas to make further processing, data sharing, or other use of the data bedone in a more efficient manner.

FIG. 15 is a block diagram of an example process performed on the targetsignature 1300. During field measurements 1505, the target signatures1300 are captured by the magnetometers 400 of the gradiometers 230. Thegradiometer processors 410 perform a number of processes 1510 on thetarget signatures 1300. The processes 1510 may include a wavelettransform 1520, matched filter 1525, fuzzy logic 1530, and jointtime-frequency analysis 1535. The target signature 1300 is provided toeach of these processes for use in analysis.

The wavelength transform 1520 produces a frequency versus time table orother data representation with magnitudes determined as a function offrequency and time. The matched filter 1525 compares the targetsignature to filters with impulse response possibly matching the targetsignature, which, when matched, results in a dipole and indication ofwhere the target object is located on the person 125 passing through thevolume of interest. The fuzzy logic 1530 includes empirical rules (e.g.,item in sock) that, when matched, generates an indication, output by thefuzzy logic 1530, to have the person 125 be stopped for a search, since,for example, a person carrying a ferromagnetic object tucked in a sockis likely to be concealing a weapon.

Another form of processing is the joint time-frequency analysis 1535,which can be used to generate a contour map of the target object so thatfurther processing or a magnetic screening system operator can visuallysee the target object on a display (FIG. 1).

Each of the processes described may be performed by the gradiometerprocessor 410 at the gradiometer level. Each of the processes 410 canalso output data or information for use by the screening computers 115to further process the target object signatures 1300. Examples ofprocessing executed by the screening computers 115 is a neural networkor polynomial decision tree 1540 that can classify the target objectinto one of multiple classes, such as a dangerous object, non-dangerousobject, or unknown object. This can be done by determining a percentageof match of a large amount of uncharacterized data to known signaturesstored in a local database. The result from the processing 1515 by thescreening computer 115 is an indicator 1545, such as text (e.g., “gun”),icon, color light indicator 120 (FIG. 1) or other means for alertinganother machine or security personnel. In the case of a physicalmachine, such as a turnstile or other mechanism that controls apassageway may be placed into a “locked” position so the person 125carrying the dangerous object or unknown object can be searched.Otherwise, the turnstile or other mechanism can remain in an unlockedstate to allow the person to pass. Any other type of machine, such ascomputer, alarm system, paging system, and so forth may also receive analert signal.

FIG. 16 is a graphical diagram of the person 125 passing through theportal 105 of the magnetic screening system 100 (FIG. 1). The magneticscreening system operator 130 is standing by and observes an identifier1545 (e.g., “gun”) displayed on the system display 110. In response, themagnetic screening system operator 130 is able to stop the person 125for inspection or “pat down” to locate all ferromagnetic objects beingcarried. If the ferromagnetic object turns out to not be a gun, theoperator 130 may enter such information into a local signature databasefor future reference. This information, as described above, may beuploaded or sent to a central database for use in updating its recordsand disseminating the new signature data or information to all of themagnetic screening systems 100 to reduce false alarms.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

While this preferred embodiment of the system is totallyelectromagnetically passive, other embodiments may employ active fieldgeneration to stimulate the target objects into electro-magneticoscillations that may be detected. The active driving functions maysimply be wire loops stimulated with radio frequency pulses or antennaedesigned to produce localized fields. In any actively driven embodiment,the operating principles of the apparatus and optimization methodsremain the same.

The arrangement of the magnetometers shown in the invention has beenlinear in nature. There are no implied limitations of the location ofthe magnetometers for the screening application. The magnetometers, solong as their location is known and they are proximal to the area to bemonitored, can be in any configuration. The optimization method andoperating modes incorporated in the invention can be equally applied aseffectively to a different, non-linear distribution of the constituentmagnetometers.

1. A magnetic screening system, comprising: an arrangement ofgradiometers each including at least three magnetometers; a gradiometerprocessor in communication with the magnetometers that scales outputsfrom the magnetometers with unequal weights and combines the scaledoutputs to orient a direction of sensitivity of the respectivegradiometer toward a volume of interest; and at least one arrangementprocessor in communication with the arrangement of gradiometers thatuses the gradiometers in a collective manner to detect a target object.2. The magnetic screening systems according to claim 1 wherein thearrangement of gradiometers define a portal.
 3. The system according toclaim 1 wherein the at least one arrangement processor localizes aposition of the target object.
 4. The system according to claim 1wherein the at least one arrangement processor identifies the targetobject.
 5. The method according to claim 1 wherein the at least onearrangement processor classifies the target object.
 6. The systemaccording to claim 1 wherein the arrangement of gradiometers define atleast one boundary of the volume of interest.
 7. The system according toclaim 1 wherein each gradiometer senses a local disturbance in amagnetic field in an electromagnetically passive manner.
 8. The systemaccording to claim 1 further including a target object signaturedatabase, and wherein the at least one arrangement processor is incommunication with the signature database and compares measurements bythe arrangement of gradiometers against signatures of target objectsstored in the signature database.
 9. A method for operating a magneticscreening system, comprising: applying unequal weights to outputs of atleast three magnetometers in a gradiometer in an arrangement ofgradiometers; combining the unequally weighted outputs to orient adirection of sensitivity toward a volume of interest; and using thegradiometers in a collective manner to detect a target object.
 10. Themethod according to claim 9 wherein the arrangement of gradiometersdefine a portal.
 11. The method according to claim 9 further includinglocalizing a position of the target object based on measurements by thegradiometers.
 12. The method according to claim 9 further includingidentifying the target object based on measurements by the gradiometers.13. The method according to claim 9 further including classifying thetarget objects based on measurements by the gradiometers.
 14. The methodaccording to claim 9 wherein orienting a direction of sensitivity towarda volume of interest includes orienting a direction of sensitivitytoward a pathway defined by a boundary including at least a subset ofgradiometers of the arrangement of gradiometers.
 15. The methodaccording to claim 9 further including sensing a local disturbance in amagnetic field in an electromagnetically passive manner.
 16. The methodaccording to claim 9 further including comparing measurements of atarget object signature by the arrangement of gradiometers to knowntarget object signatures to identify the target object.
 17. A magneticscreening system, comprising: means for applying unequal weights tooutputs at least three magnetometers in a gradiometer in an arrangementof gradiometers; means for combining the unequally weighted outputs toorient a direction of sensitivity toward a volume of interest; and meansfor using the gradiometers in a collective manner to detect a targetobject.